The Effect of Various Mixing Techniques on the Push-Out Bond Strength of Calcium Enriched Mixture

Statement of the Problem Correct proportioning and mixing are essential to ensure cements attain their optimum physical properties. Purpose The aim of this experimental study was to evaluate the influence of various mixing techniques including manual, mechanical mixing, and ultrasonic vibration on push-out bond strength of calcium enriched mixture (CEM). Materials and Method Ninety 2-mm-thick dentin disks were prepared from single-rooted human teeth and filled with CEM mixed with manual, trituration, or ultrasonic methods. Push-out bond strength values of the specimens were measured by a universal testing machine after 3 and 21 days. The samples were then examined under a stereomicroscope at 40× magnification to determine the nature of bond failure. Data were analyzed by Kruskal-Wallis and Mann-Whitney test. (p< 0.05) Results The highest (7.59 MPa) and lowest (4.01 MPa) bond strength values were recorded in conventional method (after 21 days) and trituration method (after 3 days), respectively. There was no statistically significant difference between the three techniques in 3 and 21 days. Conclusion According to the results, various mixing techniques had no effect on the push-out bond strength of CEM cement.


Introduction
Many types of dental cements are available as powder and liquid that should be mixed before application. Correct proportioning and mixing are essential to ensure that the cements attain their optimum physical properties. [1] Encapsulating along with trituration in comparison to manual mixing has the potential to reduce air spaces between adjacent particles. It results in a more thorough wetting of the powder particles and improves the unification of the resultant paste. [2] Whilst trituration uses conventional mechanical energy, there might be a potential for ultrasonic energy to be more effective. Ultrasonic vibration has a dispersing effect on the particles of materials, which frequently cluster together. Ultrasonic treatment has been reported to be effective in increasing the compressive strength, [3][4] tensile bond strength, [5][6] and hardness [7]  Calcium enriched mixture (CEM) cement was introduced in 2008 with similar clinical application to MTA but different chemical compositions. [11][12] CEM is tooth-colored water-based cement which consists of calcium oxide, calcium phosphate, calcium carbonate, calcium silicate, calcium sulphate, calcium hydroxide and calcium chloride. [13] This cement exhibited favourable results in regard to biocompatibility, antibacterial effect, and sealing properties. [13][14][15][16][17][18] There is no information about the effect of mixing techniques on the push-out bond strength of CEM cement. Therefore, the purpose of this study was to evaluate the effect of various mixing techniques including ultrasonic vibration, trituration, and manual method on push-out bond strength of CEM cement.

Materials and Method
Sixty freshly extracted human teeth including single rooted mandibular premolars or maxillary incisors that were either intact or contained only small carious lesion were used in this study. Teeth with cracks or internal resorption were excluded from the study. After removing the crowns by using a diamond disk, the middle thirds of the teeth were sectioned perpendicular to the root long axis to obtain 90 dentin disks with the thickness of 2 ± 0.2 mm. A diamond saw microtome (Mecatom T180; Presi SA, Angonnes, France) was used to obtain root dentin slices. The internal disk canals space was enlarged with Gates Glidden burs (Dentsply Maillefer; Ballaigues, Switzerland) sizes 2 to 5 to achieve a standard diameter of 1.3 mm. [9] The root sections were immersed in 17% EDTA (ethylenediaminetetraacetic acid) (Asia Chemi Teb; Tehran, Iran), and then in 2.5% sodium hypochlorite (Pakshooma; Tehran, Iran) each for three minutes to remove the smear layer. They were, then, washed with distilled water and dried. [9] The root sections were randomly divided into 6 groups (n=15), and the lumens were filled with CEM cement (BioniqueDent; Tehran, Iran) as following.
In groups 1 and 4 the CEM cement was mixed with conventional method; in groups 2 and 5, the CEM cement was mixed with trituration in an amalgamator Netherlands) at 37 C and 95% humidity for 3 days (groups 1, 2, and 3) or 21 days (groups 4, 5, and 6).

Push-out test
The push-out test was performed on the samples by using a universal testing machine (Zwick/Roell, Z050; Zwick/Roell, Ulm, Germany) ( Figure 1).

Results
The means and standard deviations (SD) of the push-out bond strength of the groups are shown in Table 1. Although the groups that CEM was mixed with trituration (group 2 and 5) showed the lowest value of push-out bond strength, no statistically significant difference was found between the three techniques in 3 and 21 days (Figure 2).

Discussion
In this study, push-out test was used to assess the bond strength between CEM cement and dentinal walls. Various methods have been described to evaluate the bonding quality of dental materials to dentin such as shear, [19][20] compressive, [4] tensile, [4,6]    To achieve optimal properties, the particles of hydraulic cements should be thoroughly mixed with water.
The mixing technique of cements is fundamental for producing effective contact between the powder particles and liquid and a final set material with optimal physical, chemical, and biological properties. [ [31][32] It should mention that the mechanical tests are unable to reflect the clinical situation; hence, future studies are required to determine the effect of these techniques on the bond strength of material in clinical applications.

Conclusion
Within the limitation of this in vitro study, it can be concluded that different mixing techniques evaluated in this study have no effect on the push-out bond strength of the CEM cement